The present invention relates in general to reduction of metal bearing salts and more particularly to production of metal powders selected from the class consisting of the valve metals, tantalum and columbium, for use in production of porous sintered anodes usable in wet or solid electrolytic capacitors.
Such powders may be produced by reacting a salt source of such metals, e.g., K.sub.2 TaF.sub.7, Na.sub.2 TaF.sub.7, Na.sub.3 TaF.sub.8, Na.sub.3 NbF.sub.8, with a reducing agent such as Na, K, Li or NaK. The reducing conditions may comprise, among other possibilities, melting the salt in a reactor and adding molten reducing agent until their respective quantities are substantially stoichiometrically matched, all the while stirring the melt; premixing stoichiometric quantities of powders of salt and reducing agent in a bomb reactor and heating up; or premelting reducing agent and stirring or mulling particles of the reducing agent therein to coat the particles of the reducing agent and then heating to reduction reaction temperatures.
The charge of valve metal containing salt may be diluted with inert eutectic fluxing ingredients such as alkali metal salts in any of the above processes to reduce the necessary temperature for holding the charge in molten state taking account of reaction products (excepting end product metal) as well as starting materials.
After a hold period at melting temperatures for about one-half to two hours, the stirring is discontinued and the reaction products are cooled. Metal powder is removed from the now solid mass by crushing and leaching.
It is an important object of the present invention to provide metal powders having usable and uniform high surface area in high yields.
It is a further object of the invention to provide a metal powder selected from the group consisting of tantalum and columbium which has high capacitance and low leakage.
It is a further object of the invention to produce powder having good handling properties for anode production consistent with one or more of the preceding objects.
It is a further object of the invention to limit reduction reaction feed molar dilution ratio to 4:1 or less consistent eith one or more of the preceding objects.
It is a further object of the invention to obtain high yield of high surface area powder consistent with one or more of the preceding objects.
It is a further object of the invention to enhance controllibility of the reduction reaction consistent with one or more of the preceding objects.
It is a further object of the invention to eliminate the need for thermal agglomeration consistent with one or more of the preceding objects.
It is a further object of the invention to provide high metal production per reduction run consistent with one or more of the preceding objects.
It is a further object of the invention to eliminate the need for size sorting above 3 microns consistent with one or more of the preceding objects.